Phys102 Lecture 18/19 Electromagnetic Induction and Faraday’s Law Key Points Induced EMF Faraday’s Law of Induction; Lenz’s Law References 21-1,2,3,4,5,6,7.

Induced EMF Almost 200 years ago, Faraday looked for evidence that a magnetic field would induce an electric current with this apparatus: Figure 29-1. Faraday’s experiment to induce an emf. He found no evidence when the current was steady, but did see a current induced when the switch was turned on or off.

Induced EMF Figure 29-2. (a) A current is induced when a magnet is moved toward a coil, momentarily increasing the magnetic field through the coil. (b) The induced current is opposite when the magnet is moved away from the coil ( decreases). Note that the galvanometer zero is at the center of the scale and the needle deflects left or right, depending on the direction of the current. In (c), no current is induced if the magnet does not move relative to the coil. It is the relative motion that counts here: the magnet can be held steady and the coil moved, which also induces an emf.

Faraday’s Law of Induction Faraday’s law of induction: the emf induced in a circuit is equal to the rate of change of magnetic flux through the circuit: or

The Magnetic Flux The magnetic flux is analogous to the electric flux – it is proportional to the total number of magnetic field lines passing through the loop. Figure 29-4. Magnetic flux ΦB is proportional to the number of lines of B that pass through the loop.

Lenz’s Law The minus sign in Faraday’s law gives the direction of the induced emf: A current produced by an induced emf moves in a direction so that the magnetic field it produces tends to restore the changed field. or: An induced emf is always in a direction that opposes the original change in flux that caused it.

Conceptual Example : Practice with Lenz’s law. In which direction is the current induced in the circular loop for each situation? Solution: a. Pulling the loop to the right out of a magnetic field which points out of the page. The flux through the loop is outward and decreasing; the induced current will be counterclockwise. b. Shrinking a loop in a magnetic field pointing into the page. The flux through the loop is inward and decreasing; the induced current will be clockwise. c. N magnetic pole moving toward the loop into the page. The flux through the loop is inward and increasing; the induced current will be counterclockwise. d. N magnetic pole moving toward loop in the plane of the page. There is no flux through the loop, and no induced current. e. Rotating the loop by pulling the left side toward us and pushing the right side in; the magnetic field points from right to left. The flux through the loop is to the left and increasing; the induced current will be counterclockwise.

i-clicker: Moving Wire Loop I A wire loop is moving through a uniform magnetic field. What is the direction of the induced current? A) clockwise B) counterclockwise C) no induced current x x x x x x x x x x x x Answer: 3

i-clicker: What is the direction of the induced current if the B field suddenly increases while the loop is in the region? A) clockwise B) counterclockwise C) no induced current x x x x x x x x x x x x Answer: 3

i-clicker A wire loop is moving through a uniform magnetic field that suddenly ends. What is the direction of the induced current? A) clockwise B) counterclockwise C) no induced current x x x x x Answer: 1

i-clicker: Shrinking Wire Loop If a coil is shrinking in a magnetic field pointing into the page, in what direction is the induced current? A) clockwise B) counterclockwise C) no induced current Answer: 1

EMF Induced in a Moving Conductor This is another way to change the magnetic flux: Figure 29-12a. A conducting rod is moved to the right on a U-shaped conductor in a uniform magnetic field B that points out of the page. The induced current is clockwise.

The induced emf has magnitude This equation is valid as long as B, l, and v are mutually perpendicular (if not, it is true for their perpendicular components).

Example : Does a moving airplane develop a large emf? An airplane travels 1000 km/h in a region where the Earth’s magnetic field is about 5 x 10-5 T and is nearly vertical. What is the potential difference induced between the wing tips that are 70 m apart? Solution: E = Blv = 1 V.

Example: Force on the rod. To make the rod move to the right at speed v, you need to apply an external force on the rod to the right. (a) Explain and determine the magnitude of the required force. (b) What external power is needed to move the rod? The resistance of the whole circuit is R. Solution: a. The external force needs to be equal and opposite to the magnetic force (IlB) if the rod is to move at a constant speed. I = Blv/R, so F = B2l2v/R. b. The external power is Fv = B2l2v2/R, which is equal to the power dissipated in the resistance of the rod (I2R).

Electric Generators A generator is the opposite of a motor – it transforms mechanical energy into electrical energy. This is an ac generator: The axle is rotated by an external force such as falling water or steam. The brushes are in constant electrical contact with the slip rings. Normal of area θ Figure 29-15. An ac generator.

If the loop is rotating with constant angular velocity ω, the induced emf is sinusoidal: For a coil of N loops, Figure 29-16. An ac generator produces an alternating current. The output emf E = E0 sin ωt, where E0 = NABω.

Example: An ac generator. The armature of a 60-Hz ac generator rotates in a 0.15-T magnetic field. If the area of the coil is 2.0 x 10-2 m2, how many loops must the coil contain if the peak output is to be E0 = 170 V? Solution: N = E0/BAω = 150 turns. Remember to convert 60 Hz to angular units.

Eddy Currents Induced currents can flow in bulk material as well as through wires. These are called eddy currents, and can dramatically slow a conductor moving into or out of a magnetic field. Figure 29-21. Production of eddy currents in a rotating wheel. The grey lines in (b) indicate induced current.

Transformers This is a step-up transformer – the emf in the secondary coil is larger than the emf in the primary: Figure 29-24. Step-up transformer (NP = 4, NS = 12).

i-clicker Transformers I A) 30 V B) 60 V C) 120 V D) 240 V E) 480 V What is the voltage across the lightbulb? 120 V Answer: 3

i-clciker Transformers II A 6 V battery is connected to one side of a transformer. Compared to the voltage drop across coil A, the voltage across coil B is: A) greater than 6 V B) 6 V C) less than 6 V D) zero A B 6 V Answer: 4

Transmission of Power Figure 29-26. The transmission of electric power from power plants to homes makes use of transformers at various stages.

Example: Transmission lines. An average of 120 kW of electric power is sent to a small town from a power plant 10 km away. The transmission lines have a total resistance of 0.40 Ω. Calculate the power loss if the power is transmitted at (a) 240 V and (b) 24,000 V. Solution: a. The total current is 120 kW/240 V = 500 A. Then the power loss is I2R = 100 kW. b. Same reasoning, different numbers: the current is 5.0 A, and the power loss is 10 W. This is why electricity is transmitted at very high voltages.